1. Field of the Invention
The present invention generally relates to a mobile broadcast system. More particularly, the present invention relates to a multi-stream reception apparatus and method for receiving data streams based on Lightweight Application Scene Representation (LASeR).
2. Description of the Related Art
Lightweight Application Scene Representation (LASeR) is a format of multimedia content for a simple multimedia service in terminals such as mobile phones suffering from resource shortages. The LASeR may be a sub-set of a Moving Picture Experts Group-4 Binary Format for Scene (MPEG-4 BIFS). The BIFS is a scene description standard for all multimedia contents and the LASeR is a scene description standard for multimedia terminals such as mobile phones in which a display size and a network bandwidth are small.
The BIFS is used for an object-based system. In the object-based system, multimedia is a set of objects. Accordingly, temporal and spatial information of each of the media needs to be indicated. For example, in the case of a weather forecast, four objects such as a weather caster, a weather chart displayed behind the weather caster, speech of the weather caster, and background music can be considered. When multiple objects are present independently, an appearing time, disappearing time and position of each object should be defined to describe one scene. This definition is the BIFS. Because related information is stored in a binary file according to the BIFS, the storage capacity is reduced.
However, the BIFS has a large amount of data of about 200 pages as described in the MPEG-4 system standard document (International Organization for Standardization/International Electro-technical Commission (ISO/IEC) 14496-1). Thus, in communication environments suffering from the shortage of available resources as in mobile terminals, there is a difficulty in using the BIFS. An alternative plan is to use the LASeR. The LASeR is the technology developed for free representations of various multimedia and interactions with users in mobile terminals with limited memory and power by performing multiplexing and synchronization processes for different elementary streams (ESs) such as a LASeR scene description, video, audio, image, font, and metadata, and minimizing complex requirements. The ESs have the above-described generic meaning. Herein, the ESs are considered as individual logical channels configuring multiple channels to be displayed.
The LASeR ES is constructed with an access unit (AU) including a command. The command is used to change a scene characteristic at a particular time. Simultaneous commands are grouped in one AU. The AU may be one scene image, short sound, one graphic image, or short animation. The commands are Insert, Replace, Delete, Background, Save, Clean, and the like. The LASeR may be referred to as the standard needed to express a commercial film (CF) in which images vary with very complex elements. A LASeR browser displays ESs in designated order. The hierarchy is present between the ESs. The LASeR browser displays scenes in order of: (1) packet reception, (2) packet decoding (AU-by-AU recovery of one image, one video clip, and the like), (3) command execution, (4) audio/video object rendering, (5) user interaction (selection, dragging, and the like), and (6) local connection or connection with an external information source.
FIG. 1 illustrates a decoder model of an MPEG-4 system to which the LASeR is applied.
Referring to FIG. 1, a demultiplexer 110 of a mobile terminal demultiplexes multiplexed logical channels on which data streams are transmitted from a service provider through a predetermined communication network. The multiplexed logical channels are divided and stored in decoding buffers 120 and are decoded by decoders 130. Decoded media are stored in composition memories 140. A compositor 150 displays the stored decoded media on a screen. To simultaneously decode the channels, the decoder model of FIG. 1 should be provided with the decoding buffers 120 for the streams, the decoders 130, and the composition memories 140. However, when the decoding buffers 120, the decoders 130, and the composition memories 140 are provided on a stream-by-stream basis, it is difficult for resources to be secured because the mobile terminal is a narrow-bandwidth communication medium.
FIG. 2 illustrates a decoder model of a mobile terminal for addressing the problem as described with reference to FIG. 1.
In FIG. 2, a demultiplexer 210, decoding buffers 220, a single decoder 230, a single composition memory 240 and a compositor 250 are provided. When the single decoder 230 and the single composition memory 240 are provided, the decoder 230 sequentially accesses and decodes data of a decoding buffer 221 during a predetermined time. When the decoding operation for the decoding buffer 221 ends, the decoder 230 repeats an operation for accessing the next decoding buffer (for example, a decoding buffer 222). In this case, a problem will be described with reference to a system decoder model according to a decoding time stamp (DTS) and a composition time stamp (CTS) of FIG. 3. FIG. 3 illustrates the decoder model according to the conventional DTS and CTS.
In FIG. 3, the DTS stands for a time stamp indicating the time at which the decoding operation ends and the CTS stands for a time stamp indicating the time at which decoded media are displayed on the mobile terminal. Because the DTS and CTS information is basic information at the time of video coding, the mobile terminal can detect the DTS and CTS information at a decoding time.
Because the single decoder 230 sequentially processes data of multiple decoding buffers 220, a problem may occur when the decoding buffers 220 are managed.
First, when the DTS is equal to the CTS, decoded media can be displayed through a compositor 250 at an appropriate time. However, when a CTS 310 is later than a DTS 320 (that is, DTS(AU1) 320<CTS(AU1) 310), the decoded media cannot be displayed during a predetermined time. In contrast, when a CTS 330 is earlier than a DTS 340 (that is, DTS(AU10) 340>CTS(AU10) 330), decoded media capable of being displayed are absent in a composition memory 240 during a predetermined time and therefore the decoded media cannot be displayed.
FIG. 4 illustrates states of decoding buffers when a single decoder and a single composition memory are used.
The decoder 430 sequentially accesses and decodes data of each of the decoding buffers 420 during a predetermined time and repeats an operation for accessing the next decoding buffer. The data is stored in a composition memory 440 and a compositor 450 displays the stored decoded data. A frame indicated by a black box is a frame (for example, I-frame) capable of being immediately decoded in each stream. When the decoder 430 accesses an arbitrary buffer, the frame capable of being immediately decoded in the decoding buffers 420 may not be placed in an access point, that is, a front end thereof. In this case, there is a problem in that an unnecessary delay time occurs until the frame capable of being immediately decoded is placed in a front end of the decoding buffers 420.
Accordingly, there is a need for an improved apparatus and method for receiving multiple streams in a mobile broadcast system that prevents a delay in time of a frame to be placed in a front end of decoding buffers.